signed: allow checks like: if (foo < 0 || foo > x) { for unsigned "foo"

signed: allow checks like:  if (foo < 0 || foo > x) { for unsigned "foo"

This silences 48 warnings on my kernel build.  It could do more.  Here are
somethings it still warns about:
   if (foo < 0 || foo > x || something else)
   if (foo > x || foo < 0)
   if ((foo < 0) || (foo > x))

It's the first type of false positive that bothers me the most.  I don't
like when people code in the 2 other styles and handling them seems
tricky...

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>